Effluent treatment

ABSTRACT

A method for treating photographic effluent to remove harmful pollutants and which reduces silver and iron ion concentrations in the effluent to no more than about 2 ppm is disclosed. The method includes an oxidation step to convert reducing sulfur compounds to oxidized products; a heavy metal ion precipitation step; a step of adding a developing agent absorbent and a sequestering agent absorbent; and a step of separating a produced solid phase from a liquid phase. The separated Liquid phase is environmentally acceptable.

This invention relates to effluent treatment, and in particular to theremoval of polluting materials from photographic effluent to render itmore environmentally acceptable, by the sequential treatment of theeffluent with chemical reagents which will cause differentialprecipitation of the pollutants.

The overflow from a photographic processor will typically consist ofpolluting substances such as heavy metal ions, sequestering agents,substances that have a high oxygen demand and colour developing agentsand their derivatives, straight disposal of which contravenes sewerregulations in many parts of the world. It is therefore essential thatthe overflow is converted by chemical or physical means to a liquidwhich can be poured into the sewer and/or a residual solid which can beremoved to a place of safe and legal disposal.

It is known that addition of an oxidising agent to liquid effuent willreduce the chemical oxygen demand (COD) of the mixture by converting thereducing sulphur compounds to their oxidised form by destruction ofthiosulphate and by precipitation of silver salts, as described in theProceedings of the SPSE conference on "Environmental Issues inPhotofinishing" 1987. Furthermore, Japanese Patent No. 51099854describes the use of an oxide, hydroxide or water soluble salt of analkaline earth metal to precipitate heavy metal ions in a photographicwaste effluent.

The use of activated carbon to remove colour developing agents, and ofan ion-exchange resin to remove a sequestering agent, such as ethylenediamine tetraacetic acid (EDTA) is also established in the art.

However, nowhere has there been described or even suggested that thesesteps may be combined into a sequential effluent treatment, preferablyeffected in a single vessel, in which polluting materials are removedfrom a liquid chemical effluent by adding reagents that sequentiallycause different pollutants or their derivatives to precipitate. A timedelay may be necessary between additions to allow one reaction tocomplete before another is started, if the reactions will or mightinterfere. Such a differential precipitation technique has manyadvantages in that the mechanical means, number of steps for separationsand total process times are all reduced by removing transfer andseparation procedures. Treatment in one vessel is less expensive thanusing individual units, allows a small compact module for treating theeffuent at small processing sites and results in a small mass and volumeof easily disposable solid wastes.

The process can be applied to the mixed overflows from all the baths inthe photographic processing machine or to the overflow from a singlebath that is contaminated by more than one polluting substance.

The effluent to be treated may, for example, have arisen from any of thephotographic processes described in Item 308119, Research DisclosureDecember 1989, Industrial Opportunities Ltd., Hants., U.K., andespecially sections XIX, XX and XXIII thereof.

The one-vessel method would be applicable furthermore to any liquidchemical effuent that was deleterious to the environment providing achemical or physical means of precipitating the damaging componentscould be found.

The solids produced from the above reactions can be removed by somemechanical means in one process step, such as centrifugation orfiltration, in particular as described in co-pending U.K. applicationNo. 9019607.2, filed on even date herewith, in which an inert powder ofhigh bulk increases the effectiveness and rate of filtration. Preferablythis powder may be added during sequential precipitation treatmentrather than as a final stage of the effluent treatment.

According to the present invention therefore there is provided a methodof treating photographic effluent by precipitating or otherwise removingharmful pollutants to provide an environmentally acceptable liquid phaseand a disposable solid phase. Preferably the separation is carried outby differential precipitation of the pollutants through sequentialaddition of chemical reagents.

The order in which the different precipitants are added may be varied ormore than one precipitant may be added simultaneously. However foreffective treatment it is advisable to add the oxidising agent as theinitial stage. Conveniently, a short period is then allowed for thenecessary reactions to occur, otherwise the oxidising agent may bedecomposed before it reacts with the other solids present. The alkali,activated carbon and, where used, the inert powder of high bulk to aidfiltration may then be added separately or at the same time, followedfinally by the addition of the component to remove the sequesteringagent.

Suitable oxidising agents may be inorganic or organic and may includepermanganate, perchromate, persulphate, perphosphate, perborate orpercarbonate, benzyl or urea peroxide but conveniently hydrogen peroxideis used, usually 30%, preferably in an amount of 20 to 100 ml/l , mostpreferably 30 to 50 ml/l of effluent.

Alternatively it is sometimes useful to have the oxidant as a solidsince this is easier and safer to dose by hand or can also be dispensedby similar automatic means to the other solids in the process. Inparticular sodium percarbonate is especially useful as it has a highoxygen content and has the added benefit of reducing the amount ofcalcium hydroxide required in the step as detailed hereunder, by natureof its being alkaline.

The heavy metal ions are removed by the addition of an excess of a metalhydroxide, oxide or carbonate, preferably a Group II metal hydroxide,such as zinc, cadmium, mercury, magnesium or most preferably calciumhydroxide to the resulting precipitate. This alkali may be used in theform of a solution, powder, slurry or as pellets and sufficient must beadded to render the effluent alkaline. Generally an amount of from 20 gto 90 g/liter effluent has been found to be suitable. Addition of analkaline earth compound also precipitates most of the sulphur compoundsas the sulphate and dithionates are converted to their insolublecounterparts for subsequent removal.

The activated carbon, in an amount of 15 to 30 g/l effluent, forremoving the colour developing agent and the inert powder for assistingfiltration may be added prior to the alkaline addition, coterminously orsubsequently but conveniently all three reagents may be addedsimultaneously. The inert powder may be aluminium hydroxide, adiatomaceous earth such as kieselguhr or even wet sawdust, preferably inthe range from 10 to 100 g/l effluent, most preferably 25 to 35 g/liter.

The sequestering agent such as EDTA or propylene diamine tetraaceticacid (PDTA) may be removed by the subsequent addition of an ion-exchangeresin, such as Zerolit TM FFip, preferably in an amount of 75 to 125g/liter effluent which also reduces the pH by partial removal ofhydroxide ions. Alternatively and preferably the salt of a Group III ortransition metal such as aluminium sulphate may be added, as disclosedin co-pending U.K. application No. 9019606.4, filed on even dateherewith, in which the metal complex is precipitated from alkalinesolution whilst restoring the effluent to a more neutral pH without theaddition of extra acid.

The precipitate can then be removed by some mechanical means such asfiltering, settling or centrifugation to provide an aqueous phase whichis considerably more environmentally acceptable and a solid waste whichis readily disposable.

The invention will now be illustrated with reference to the followingExample which does not in any way limit the scope of the invention.

EXAMPLE 1 Sequential Precipitation

A model processor effluent was made by combining 300ml of the followingdeveloper/amplifier with 500ml of a bleach-fix, also described below:

    ______________________________________                                        Developer/Amplifier:                                                          potassium carbonate        20    g                                            diethylhydroxylamine       5     g                                            ethylenediaminetetraacetic 1     g                                            acid (EDTA)                                                                   Colour developer CD3       4     g                                            30% hydrogen peroxide      5     g                                            water to                   1     liter                                        pH adjusted to 10.3 with sulphuric acid or NaOH                               Bleach Fix:                                                                   sodium iron (III) EDTA     20    g                                            sodium thiosulphate        50    g                                            sodium sulphite            20    g                                            acetic acid                20    ml                                           water to                   1     liter                                        pH adjusted to 5.5 with acetic acid                                           ______________________________________                                    

0.8 g of silver chloride was added to this mixture and this was stirreduntil it had dissolved. The mixture was left to stand overnight to allowmost reactions to complete.

100ml of this model effluent were poured into a 250 ml beaker. To thiswas added 4 ml of hydrogen peroxide to reduce the oxygen demand of themixture by converting the reducing sulphur compounds to their oxidisedform. This mixture was stirred for two minutes to allow the reactions tocomplete. 2.8 g of calcium hydroxide were then added to the mixture toprecipitate the iron and most of the oxidised sulphur compounds.Following this, 2 g of activated charcoal was added to remove colourdeveloping agents and their derivatives, and also thediethylhydroxylamine. After one minute stirring, 10 g of Zerolit TM FFipion-exchange resin was added to remove the sequestering agent, EDTA, andalso to reduce the pH by partial removal of hydroxide ions.

The mixture was filtered by standard means using fast filter paper andthe filtrate was analyzed.

All additions were carried out at room temperature with no deliberatetemperature or pressure control.

Table 1 gives the concentrations of components before and aftertreatment.

                  TABLE 1                                                         ______________________________________                                                    Concentration Concentration                                       Component   before treatment                                                                            after treatment                                     ______________________________________                                        Iron        1875 ± 300                                                                           ppm     <2 ± 2                                                                              ppm                                    EDTA        9375 ± 500                                                                           ppm     <100 ± 100                                                                          ppm                                    Colour Developer                                                                          1100 ± 50                                                                            ppm     <10 ± 10                                                                            ppm                                    (CD.sub.3)                                                                    Chemical Oxygen                                                                           10 ± 2 g/liter <1 ± 1                                                                              g/liter                                Demand                                                                        Silver      713 ± 10                                                                             ppm     2 ± 1 ppm                                    ______________________________________                                    

Table 1 shows that all the pollutants measured are removed to an extentthat they cannot reliably be detected by the analytical methodsemployed.

EXAMPLE 2

The above process was repeated using, however, as oxidising agent 4gsodium percarbonate instead of the 4 ml hydrogen peroxide, therebyreducing to 1.5 g the amount of calcium hydroxide required toprecipitate the iron and most of the oxidised sulphur compounds.

The concentrations of components after treatment were again as given inTable 1.

I claim:
 1. A method of treating a photographic effluent to removeharmful pollutants, including heavy metal ions comprising silver andiron ions, comprising:(a) providing a vessel containing a liquidphotographic effluent that includes harmful pollutants; (b) introducinginto said vessel an oxidizing agent selected from the group consistingof hydrogen peroxide, a percarbonate, a permanganate, a perchromate, apersulfate, a perphosphate, or a perborate to convert reducing sulfurcompounds to oxidized products; (c) after a period of time sufficientfor said oxidizing agent to convert reducing sulfur compounds tooxidized products, introducing into said vessel chemical reagentscomprising a heavy metal ion precipitant selected from a metalhydroxide, oxide or carbonate, a developing agent absorbent, and asequestering agent absorbent, thereby producing a mixture comprising asolid phase and an environmentally acceptable liquid phase; (d)separating and removing said solid phase from said liquid phase, whereinsaid liquid phase has silver and iron ions in a concentration of each ofno more than about 2ppm; and (e) disposing of said solid phase.
 2. Amethod according to claim, 1 wherein said oxidizing agent is hydrogenperoxide.
 3. A method according to claim 1 wherein said oxidizing agentis sodium percarbonate.
 4. A method according to claim 1 wherein saidheavy metal ion precipitant comprises a Group II metal hydroxide.
 5. Amethod according to claim 4 wherein said Group II metal hydroxide iscalcium hydroxide.
 6. A method according to claim 1 wherein saiddeveloping agent absorbent is activated carbon.
 7. A method according toclaim 1 wherein said sequestering agent absorbent comprises anion-exchange resin.
 8. A method according to claim 1 wherein saidsequestering agent precipitant comprises a Group III or transition metalsalt.
 9. A method according to claim 8 wherein said metal salt isaluminum sulfate.
 10. A method according to claim 1 wherein saidprecipitant and said absorbent are introduced sequentially.
 11. A methodaccording to claim 1 wherein said precipitant and said absorbent areintroduced simultaneously.
 12. A method according to claim 1 whereinsaid separating and removing said solid phase from said liquid phase iscarried out by filtration, settling, or centrifugation.
 13. A methodaccording to claim 1 further comprising adding an inertfiltration-assisting powder prior to said separating and removing saidsolid phase from said liquid phase.
 14. A method according to claim 1wherein said oxidizing agent is hydrogen peroxide or sodiumpercarbonate, said heavy metal ion precipitant is a Group II metalhydroxide, and said developing agent absorbent is activated carbon, saidmethod further comprising introducing into said vessel, following theintroduction of said chemical reagents and prior to separating andremoving said solid phase from said liquid phase, a sequestering agentabsorbent comprising an ion-exchange resin.